Industry has used mechanical seals to replace packing as a means of sealing a rotary shaft or body passing through the wall of a vessel containing liquid or gas. As will be appreciated by those skilled in the art, a mechanical seal is basically formed of two elements or rings. One of the seal rings may be connected to the rotating body while the other ring is sealed to the housing. The radial end faces of the seal rings are arranged in an opposed relationship relative to one another and, thus, such seals are often referred to as end face seals. To allow for wear, at least one seal face may be spring loaded and axially movable relative the other seal face. In many applications, one of the seal faces is comprised of a softer self-lubricating material, such as carbon, while the other seal ring face will be a hard material, such as metal, ceramic, or metal carbide. Moreover, to accommodate high pressure applications, the faces of the seal rings are usually hydraulically balanced.
Because of the seal rings' relatively high cost, wear of the seal faces is an important consideration. Therefore, in recent years, many seal rings are designed such that both sealing faces are made from hard materials to accommodate more severe service. Because such hard materials are less yielding and have poor self-lubricating qualities, it is vital that a very thin lubricating fluid film, measurable in microinches, must be maintained between the seal faces.
The end face type seal has proven very effective and industry has widely adopted its use. Difficulties, however, have been encountered when end face type seals are used under high pressure and particularly under variable high pressures. Excessive leakage and erratic performance have been experienced even when the seals are manufactured with the highest possible manufacturing precision. As machine operating pressure increases, distortion of the seal rings can, and often does, result. Seal ring distortion, although measurable in only microinches, causes the lubricating fluid film between the seal rings to either collapse, resulting in zero film thickness, or such distortion causes the film to be too thick, thus causing leakage between the seal faces.
Tests have revealed a corrolation between fluid film thickness and the relative relationship of the seal faces. In particular, if the opposed seal faces are flat and parallel, the movable seal face will be urged into contact with and across the interface of the stationary or fixed seal ring. Consequently, the lubricating fluid film required therebetween will collapse resulting in zero film thickness and rapid seal wear. To prevent this from occuring, the opposing faces of the seal rings are specifically designed in a nonparallel relationship relative to one another. That is, the seal faces converge radially in the direction from the high pressure side to the low pressure side of the seal. By such construction, the pressurized product fluid has access to the convergent opening between the seal faces. The pressurized product fluid between the seal faces provides a sufficiently large opening force tending to move the resiliently mounted seal face away from the fixed seal face. The magnitude of this opening force will be a function of the size of the gap between the seal faces.
Because of the above, mechanical seals are often designed and manufactured with their sealing faces having a predetermined angle of convergence relative one another during steady state operation. As may be appreciated, heretofore conventional mechanical seals have been designed very carefully. In designing seals, it is the art of the seal designer to anticipate thermal and mechanical seal ring deformation and distortion, seal ring cross section, as well as width and balance of the faces to produce an acceptable film thickness under conditions of steady state operation. Additionally, seal designs are often specifically suited to predetermined operating conditions regarding temperature pressure, speed, load and fluid characteristics.
Today's industrial applications, however, vary considerably. Some applications often involve pumping unusual liquids which verge on being gases at similar densities. That is, the operating parameters of a pump regarding pressure speed, load and other considerations often changes. To accomodate such changing conditions, the faces and seal ring cross sections have to be repeatedly modified to adapt the pump to the particular operation. Even with carbon seal rings, such changes are costly. The rings made from the new and harder materials are even more expensive to modify and are far less forgiving of slight variations from optimum shape.
U.S. Pat. 3,275,330, issued to E. N. Rein on Sept. 27, 1966, discloses a face-type mechanical seal which is specifically designed to minimize distortion in a seal ring. To effect that end, the seal ring cross-section of the Rein device attempts to cancel any appreciable unbalance of forces which tend to distort the contacting seal faces out of a true flat condition. To achieve the desired cross-section is both a delicate and costly procedure.
Therefore, there remains a need in the art for a controllable seal wherein the fluid film thickness between the separated seal faces can be controlled despite seal ring deformation and without separate and specific design configurations for each pump operating condition. By accepting seal ring deformation and controlling same, the angle of convergence between the seal faces can be optimized for a wide range of operating conditions.